1. Field of the Invention
The present invention relates to an organic electrolytic solution type cell comprising a non-aqueous electrolytic solution such as a lithium cell.
2. Description of the Related Art
As the non-aqueous electrolytic solution for an organic electrolytic solution type cell, a solution of a lithium salt in a polar solvent is used. Examples of the lithium salt are LiClO.sub.4, LiAsF.sub.6 and LiPF.sub.6, and examples of the polar solvent are propylene carbonate, .gamma.-butyrolactone, dimethoxyethane and dioxolane.
In the cell comprising the non-aqueous electrolytic solution, a negative electrode Li reacts with a harmful material such as water, oxygen gas, nitrogen gas and impurities which are contained in the electrolytic solution or, in some case, components of the electrolytic solution, so that the surface of the lithium electrode is deactivated. Thus, cell performance, such as closed circuit voltage, are deteriorates during storage.
When the organic electrolytic solution type cell is used as a secondary cell, the reaction between the harmful materials in the electrolytic solution and the lithium electrode deteriorates the charge-discharge cycle characteristics.
LiPF.sub.6 is practically preferable as an electrolyte since it is safe, unlike LiClO.sub.4, or it is a toxic, unlike LiAsF.sub.6. However, it is thermally unstable and decomposes during storage at a high temperature. Therefore, the cell performance is further deteriorated.
To improve the thermal stability of LiPF.sub.6, it is proposed to add hexamethylphosphoric triamide (hereinafter referred to as "HMPA") or tetramethylethylenediamine (hereinafter referred to as "TMEDA"). Since such additives do not achieve sufficient effects and HMPA may react with active lithium metal of the negative electrode, improvement of the storage stability cannot be expected.